Memory cells based on ferromagnetic structures are known to the art. These cells are constructed from a three layered structure in which two ferromagnetic layers are separated by a conducting non-magnetic layer. The ferromagnetic layers have different degrees of "hardness". For the purposes of this discussion, the "hardness" of a ferromagnetic layer will be defined to be the magnetic field needed to change the direction of magnetization of the material, a "hard" layer requiring a greater magnetic field than a "soft" layer. Data is stored in one of the layers by magnetizing the layer, a "1" corresponding to one direction of magnetization, a "0" to the other direction of magnetization. The other layer is used as a "reference" layer. For the purposes of this discussion, it will be assumed that data is stored in the soft layer, and the hard layer is used as the reference layer. In this case, the reference layer is permanently magnetized in one direction, and a field less than that needed to switch the reference layer is applied to store data in the soft layer.
The data is read by measuring the current that flows through the structure when a potential is applied across the two ferromagnetic layers. If the data layer is magnetized in the same direction as the reference layer, the device displays a smaller resistance to current flow than the case in which the two layers have different directions of magnetization.
While the basic cell has been known for some time, useful memories based on these cells have not been practical. Conventional memory architectures in which the memory cells are isolated or connected from a bit line by a transistor cannot be used with ferromagnetic memory cells because the resistance of the memory cells is small compared to the resistance of a transistor in the conducting state. Hence, the changes in resistance of the memory cell are masked by the high resistance of the isolation transistor.
To write a memory cell, a magnetic field must be applied to that memory cell that is sufficient to switch the soft layer, but less than the field that switches the reference layer. In addition, neighboring cells must not be switched. This requires a system for generating a local magnetic field at each memory cell. Memory architectures used in conventional CMOS memories or ferroelectric-based memories do not provide a means for generating local magnetic fields. In addition, conventional memory architectures require transistors in series with the memory elements to isolate the memory elements from the lines used to write the data therein. These transistors limit the currents, and hence, the magnetic fields, that may be applied to the memory elements in a magnetic memory cell.
Broadly, it is the object of the present invention to provide a memory system based on ferromagnetic memory cells.
It is a further object of the present invention to provide a memory system that does not require each cell to be isolated or connected by a pass transistor.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.